Normal peripheral blood contains mature red blood cells which are free of nucleus. Nucleated red blood cells (NRBC), or erythroblasts, are immature red blood cells. They normally occur in the bone marrow but not in peripheral blood. However, in certain diseases such as anemia and leukemia, nucleated red blood cells also occur in peripheral blood. Therefore, it is of clinical importance to measure NRBC. Traditionally, differentiation and enumeration of nucleated red blood cells are performed manually. The process involves the smearing of a blood sample on a microscope slide and staining the slide, followed by manual visual analysis of the individual slide. The nucleated red blood cell concentration is reported as numbers of NRBC per 100 white blood cells (WBC). Usually, 200 white blood cells and the numbers of NRBC present in the same region on a blood smear are counted and the numbers are divided by 2 to express the NRBC concentration as the numbers of NRBC/100 WBC. This approach is extremely time-consuming as well as being subjective to the interpretation of the individual analyzing the slide.
In recent years, several fluorescence flow cytometry methods have been developed for differentiating nucleated red blood cells. These methods utilize nuclear specific staining techniques to distinguish nucleated red blood cells from other cell types because it is difficult to differentiate nucleated red blood cells only based on their electronic or optical properties.
U.S. Pat. No. 5,298,426 (to Inami et al.) discloses a fluorescence method for differentiating nucleated red blood cells. The method utilizes a two-step staining using a first fluid and a second fluid. Inami et al. teaches that the first fluid contains an erythroblast-staining dye that diffuses into nucleated red blood cells to specifically stain their nuclei, and then separating a group of nucleated red blood cells from other cell groups on a two-dimensional plot whereby the results of NRBC differentiation are computed.
U.S. Pat. No. 5,559,037 (to Kim et al.) discloses a method for flow cytometric analysis of nucleated red blood cells and leukocytes. The method comprises lysis of red blood cells and NRBC cytoplasm from a whole blood sample to expose the nucleated red blood cell nuclei to a vital nuclear stain and minimizing the permeation of the vital nuclear stain into the leukocytes and analyzing the sample by measuring fluorescence and two angles of light scatter. This method features a triple triggering method which blocks signals from debris (fluorescent and non-fluorescent) and identifies the signals which fall below the axial light loss (ALL) trigger but above the fluorescence trigger (FL3) as NRBCs. This method requires heating of the reagent to 42° C. in order to obtain the NRBC and leukocyte differentiations.
U.S. Pat. No. 5,648,225 (to Kim et al) discloses a method of using a multipurpose lysing reagent for subclassification of nucleated blood cells. The method comprises the steps of lysing a blood sample with the multipurpose lysing reagent which contains a nuclear stain, incubating the sample mixture at an elevated temperature, and determining the nucleated blood cells including NRBCs with an automated electro-optical hematology instrumentation.
U.S. Pat. No. 5,879,900 (to Kim et al) discloses a method of differentiating NRBCs, white blood cells (WBC), damaged white blood cells, and white blood cell subpopulations in a blood sample by flow cytometry. The method includes lysing a blood sample; staining nucleated red blood cells and any damaged white blood cells with a vital nuclear stain; analyzing the sample mixture by measuring fluorescence, axial light loss and light scatter signals from 3° to 10°; constructing a three-dimensional plot from the fluorescence and light scatter signals; and differentiating and enumerating WBC, NRBC, damaged WBC and a WBC subclass differential.
EP 1 004 880 A2 discloses reagents and a method for discrimination and counting of nucleated red blood cells. The method includes the steps of lysing red blood cells, staining white blood cells and nucleated red blood cells, assaying the sample by measuring at least one scattered light parameter, and at least one fluorescence parameter.
The above described methods enable differentiation and enumeration of nucleated red blood cells and leukocytes by combined fluorescence and light scatter measurements. However, fluorescence measurements are complex and expensive detection methods.
U.S. Pat. No. 5,874,310 (to Li et al) discloses a method for differentiation of nucleated red blood cells. The method includes lysing mature red blood cells and analyzing the sample in a flow cell by light scatter measurements to differentiate nucleated red blood cells from other cell types. The light scatter measurements are performed by using two low angle light scatter signals of less than 10°. The method further includes a concurrent differentiation of white blood cells using electronic and optical analyses, wherein the electronic analysis is a DC impedance measurement.
U.S. Pat. No. 5,917,584 (to Li et al) further discloses a method using two angles of light scatter measurements to differentiate nucleated red blood cells from other cell types, wherein the first light scatter signal is a low angle light scatter signal and the second light scatter signal is a medium angle or a right-angle light scatter signal.
U.S. Pat. No. 6,410,330 (to Li et al) also discloses a method for differentiation of nucleated red blood cells. The method includes lysing red blood cells of a blood sample with a lytic reagent, measuring nucleated blood cells by DC impedance measurement in a non-focused flow aperture, differentiating nucleated red blood cells from other cell types, and reporting nucleated red blood cells in the blood sample.
U.S. Pat. No. 6,472,215 (to Huo et al) teaches a method of differentiating nucleated red blood cells by lysing a first aliquot and a second aliquot of a blood sample separately with a first lysing reagent system and a second lysing reagent system, respectively; measuring the first sample mixture in a flow cell by DC impedance, radio frequency, and light scatter measurements; measuring cell distributions and counting remaining blood cells in the second sample mixture by DC impedance measurements in a non-focused flow aperture; analyzing blood cell distribution patterns obtained from measuring the first sample mixture and from measuring the second sample mixture respectively; and further performing a combined analysis to differentiate NRBCs from other cell types and determine numbers of NRBCs in the blood sample.
It is known that differentiation of nucleated red blood cells from other cell types, particularly white blood cells, is technically challenging because of the potential overlapping signals from other cell types, when measured by their size, and light scatter and fluorescence properties. The prior art detection systems and detection methods can be further improved in terms of cost, simplicity and efficiency of the measurement.
Furthermore, it is known that certain cell types and/or cellular materials tend to interfere with the nucleated red blood cell measurement. Depending on the detection methods used for the measurement, the interference materials can be different. For example, when fluorescence measurement is used for measuring the nucleated red blood cells, aged white blood cells tend to interfere with the measurement, because they tend to be lysed by the reagent used for preparing the sample mixture, and the nuclei of the lysed white blood cells tend to overlap with nucleated red blood cells. It has been found that when using light scatter and impedance measurements for measuring nucleated red blood cells, giant platelets, platelet clumps and sickle cells can cause interference, because these materials can overlap with nucleated red blood cells in one or more of the measurements.
Based on foregoing, there still exists a need for a simple, less costly, yet reliable detection method and apparatus for differentiating and enumerating nucleated red blood cells. There also exists a need for further improvement of accuracy of the measurement, particularly in the presence of interference materials.